SEA08LA038 · Hiller UH-12E

While conducting an aerial application flight, the pilot reported that he was initiating a turn over a set of power lines at an altitude of about 60 feet above ground level (agl). As the helicopter crossed over a series of power lines, the pilot heard a "loud pop" and felt an extreme vibration. The pilot maneuvered the helicopter away from the power lines and initiated a forced landing to an open field. He reported that control of the helicopter was "marginal" as the helicopter descended. Shortly before touchdown, the helicopter began to drift (un commanded) to the left and the pilot lowered the collective. The helicopter touched down hard, pivoted to the left and rolled over onto its right side. Post accident examination of the helicopter revealed that a control rotor had separated from the rotor system. The separated control rotor was found in the area where the vibration started. Examination of the control rotor assembly revealed the tubular spar of the blue control rotor was circumferentially fractured at the outboard cuff bolt location. Magnified examinations of the outboard fracture surfaces revealed surface features consistent with fatigue progression in two fronts around the spar. Fatigue initiation locations were identified on opposite sides of one of the outboard bolt holes in the spar. One fatigue origin was identified on the bore surface of the bolt hole and the other was mechanically damaged preventing determination of its exact location. The bore surface was rough and no obvious corrosion was noted in the hole. From the bolt hole the fatigue propagated almost entirely around the spar with very small overstress regions connecting the fatigue to the opposite bolt hole in the spar. No corrosion was noted in the originating bolt hole, however, the exterior surface of the spar exhibited wide spread pitting corrosion and fretting on the faying surfaces with the control rotor cuff. Additional localized corrosion was noted on the interior faying surface of the cuff. Maintenance records indicated that the most recent 100-hour inspection was conducted about 77 hours prior to the accident. The logbook entry associated with the inspection indicated that Airworthiness Directive AD 97-10-16 had been complied with. AD 97-10-16 (effective date 6/26/97) states, in part,: "To prevent separation of the control rotor blade assembly and subsequent loss of control of the helicopter, accomplish the following: (a) Within the next 100 hours time-in-service (TIS) after the effective date of the AD, unless previously accomplished within the last 100 hours TIS, and thereafter at intervals not to exceed 100 hours TIS from the date of the last inspection, or at the next annual inspection, whichever occurs first, inspect the blade spar tube and cuff for corrosion or cracks, or elongation, corrosion, burrs, pitting or fretting of the bolt holes, and repair, as necessary..."

Verbatim NTSB analysis and probable cause from the NTSB dataset